Overview
This lecture reviews beta blockers, focusing on their classification, mechanisms of action, effects on organ systems, clinical uses, and key side effects, with emphasis on distinctions among agents and their clinical implications.
Classification of Beta Blockers
- Beta blockers are divided into selective (mainly beta-1), non-selective (block beta-1 and beta-2), and mixed (block beta-1, beta-2, and alpha-1) agents.
- Mixed agents include carvedilol and labetalol, which have added alpha-1 blocking properties.
Mechanisms of Action
- Beta blockers decrease heart rate (negative chronotropy), conduction velocity (negative dromotropy), and force of contraction (negative inotropy).
- Blockade of beta-2 in vasculature leads to vasoconstriction, counteracting some blood pressure-lowering effects.
- Lipophilic beta blockers cross the blood-brain barrier, reducing central sympathetic tone, but may cause CNS side effects like fatigue and depression.
Effects on Organ Systems
- In the heart, beta blockers reduce oxygen demand and remodeling, beneficial in heart failure.
- In kidneys, beta-1 blockade decreases renin release, slightly reducing the renin-angiotensin-aldosterone system (RAAS) activity.
- Unlike non-dihydropyridine calcium channel blockers (verapamil, diltiazem), beta blockers also reduce cardiac remodeling.
Clinical Uses & Drug Differences
- Beta blockers are not first-line monotherapy for hypertension due to limited BP-lowering efficacy.
- Specific agents (carvedilol, metoprolol succinate) reduce mortality in heart failure and must be titrated slowly.
- Esmolol is used IV in acute settings (e.g., aortic dissection) due to a short half-life.
- Propranolol is used for migraine prophylaxis and anxiety due to CNS penetration.
Adverse Effects & Contraindications
- Common side effects: bradycardia, fatigue, depression, erectile dysfunction, bronchospasm, worsening of peripheral vascular disease.
- Contraindications: second/third degree heart block, sick sinus syndrome, uncontrolled asthma or COPD.
- Beta blockers may mask hypoglycemia signs, except for sweating.
- Lipophilic agents more likely to worsen depression and cause CNS side effects.
Key Terms & Definitions
- Chronotropy — effect on heart rate.
- Dromotropy — effect on conduction velocity through the AV node.
- Inotropy — effect on force of cardiac contraction.
- Remodeling — structural changes in the heart, often after injury.
- Lipophilic — able to cross blood-brain barrier; fat-soluble.
- Renin-Angiotensin-Aldosterone System (RAAS) — hormonal system regulating blood pressure and fluid balance.
Action Items / Next Steps
- Review the pharmacology and indications for carvedilol and metoprolol succinate in heart failure.
- Understand contraindications for beta blocker use (heart block, asthma/COPD).
- Study the side effect profiles and organ system effects for different beta blocker classes.
- Prepare for potential exam questions on clinical scenarios involving beta blocker selection and titration.